1. Field of the Invention
The present invention relates to a method for the detection of a target by a radar in the presence of noise. It can be applied especially to the detection of small sea targets moving for example in a high sea clutter.
2. Description of the Prior Art
Sea patrol missions introduce specific conditions of detection. First of all, surveillance missions require 360.degree. observation and extensive radar coverage. The radar antenna is therefore rotational and the angles of incidence are grazing so as to achieve radar ranges of more than 30 nautical miles for an altitude of about 2000 feet. Secondly, the detection is done in real time. This factor limits the available computation time and the complexity of the detection algorithm. Finally, and thirdly, the standard Doppler processing operations are often inoperative since the criterion of detection uses differences in speed between the target and the parasitic echoes, known as clutter, that are caused by the waves. Now clutter is mobile and targets such as safety boats can move according to the rhythm of the waves. Thus detection processing operations are generally done on incoherent signals.
For these reasons, the processing operations implemented in the context of sea patrols consist of a filtering adapted to incoherent signals. In particular, they cannot perform any processing or phases and use simply the energy level of the back-scattered signal to distinguish between the signals returned from the target and the signals returned from the clutter. The anti-clutter means developed thus seek to increase the signal-to-clutter ratio by reducing the size of the resolution cell. To this end, a pulse compression is performed to obtain a resolution of about 1 meter for example and the width of the antenna lobe of the radar is reduced so as to obtain a width at 3 dB of about 1.degree.. In the context of sea patrols, the detection devices take account of the period of correlation of sea clutter by setting up a criterion of rotation-by-rotation integration known as an N/M type integration. According to a criterion of this kind, a target is assumed to exist if a corresponding echo has been detected at least a given number N of times on each of a number M of rotations. The rotation speed of the antenna thus becomes a key factor in the detection. If this speed is great, the number of rotations during which the target can be detected is large but, on the other hand, the target is rapidly scanned by the antenna lobe during one rotation. The decorrelation of the clutter, rotation by rotation, is then low. To favor the detection on each rotation, it is therefore necessary to reduce the rotation speed, but the number of detection opportunities is then reduced. At present, both alternatives are used. There are therefore sea patrol radars with high antenna rotation speeds of about 200 rotations per minute and sea patrols with a moderate antenna rotation speed of about 30 rotations per minute.
However, the processing operations presently implemented do not make it possible to meet the increasingly stringent requirements laid down for sea patrols. Detection operations are henceforth expected do be done in increasingly rough seas, for example under force 4 and force 5 wind conditions for targets with increasingly small radar cross-section values, for example 1 m.sup.2 to 5 m.sup.2, and hence for increasingly small useful signal-to-clutter ratios. Furthermore, the required probabilities of false alarms may be in the range of 10.sup.-5 to 10.sup.-6. This appreciably corresponds to the bit rate that a radar operator can handle without being overwhelmed by the number of echo blips. A threshold level corresponding to these false-alarm rates is difficult to reconcile with a standard detection of small targets which produces a far greater false-alarm rate.
Apart from the fact that it is necessary to work at increasingly small signal-to-clutter ratios, the very nature of clutter at high resolution, at grazing angles of incidence and for stormy seas, is changing and itself increasing the severity of the conditions of detection. Thus, the differences between the signals back-scattered by the clutter and by a localized target are attenuated. To demarcate the problem of this detection more clearly, we may examine a simple modeling of back-scattering on the surface of the sea by inclined planes on which there are superimposed a surface with low roughness, i.e. with a small peak-to-valley height, for example equal to the radar wavelength. At these values of incidence close to the vertical or when there is poor resolution, the roughness is wiped out. It then plays a small role in the back-scattering phenomenon which depends essentially on the slope of the tangential plane. It can easily be seen that when the radar incidence becomes flat and when the resolution improves, the roughness becomes a decisive factor in back-scattering. This results in effects of masking and multiple paths that are expressed physically in the appearance of spikes that get detached from the background of low level clutter. When a test is made on the power of the signal, these spikes go beyond the detection threshold and activate false alarms. It can also be seen that these spikes are related to the breaking waves.
Certain known results make it possible to improve the detection algorithms. In particular, there is a known way by which the analysis of the incoherent signal shows that the strong echoes lengthen the distribution tail of the amplitude of sea clutter which is often represented by relationship K. More generally, the sea clutter is no longer modeled statistically by a process distributed according to a Gaussian law but rather by a compound Gaussian process, namely a Gaussian process multiplied by a random variable. A process according to a relationship K is a particular compound Gaussian process. There are also known ways wherein the analysis of a complex signal reveals that the band length of the clutter increases with the appearance of the above-mentioned spikes because the diversity of the speed of the reflecting particles related to the breaking of the waves, such as spray for example, widens the Doppler spectrum. It is also known that the Doppler signature of the breaking waves is characteristic of the appearance of fast reflectors. The reflecting structures located at a peak of the wave about to break acquires its high phase speed and then slows down in the course of time. Finally, it is known that the waves are amplitude-modulated and break when a peak passes through the maximum modulation that corresponds to the point of maximum instability. The shift in the breaking waves is therefore linked to the relative speed of the peaks with relation to the modulation while the Doppler frequency of the clutter reflects the speed of the peaks only. Thus, unlike localized targets with a low-fluctuation Doppler frequency, the tracking of a spike by the estimation of its Doppler frequency is inappropriate.
The limitations of the standard processing methods are therefore of several kinds. Among them:
the standard detection algorithms based on matched filtering, which result in a correlation and then a detection of an envelope, are optimum in Gaussian noise whereas the clutter is better represented by a compound Gaussian process; PA1 the incoherent detection does not enable any distinction between the target and the spikes for localized targets with a radar cross-section, and coherent detection seems to be better indicated when there are spikes and breaking waves since the energy is concentrated on a limited band of frequencies in the case of a localized target and is dispersed over a wider band in the case of breaking waves; PA1 the value of rotation-by-rotation integration as practised is limited because the size of the resolution cell is increased by carrying out groupings in distance, especially for problems of sizes of memory. The probability of the presence and detection of a spike without even taking account of its period of existence in a widened zone is then increased and the criterion of N/M detection suffers loss of effectiveness. The rotation-by-rotation integration makes only superficial use, in the form of a binary test for the presence or absence of a strong echo, of the fastest decorrelation of the sea clutter from rotation to rotation. It does not play a role especially in the values of changes in frequency. This binary aspect causes a deterioration in the information contained in the N rotations that corresponds to the presence of the target. PA1 a first step for the estimation of the Doppler frequency (f.sub.d) of the target; PA1 a detection step, the target being detected if an associated variable Z is greater than or equal to a predetermined threshold S, the variable Z being defined according to the following relationship: ##EQU2##